This invention relates to scanning radio receivers, and more particularly to programmable scanning radio receivers.
Early scanning receivers used crystals for tuning, and the number of channels available for reception was limited to the number of crystals in a given receiver. A receiver typically contained either eight or sixteen crystals, and different crystals had to be installed to enable reception of different frequencies, including the frequencies of other desired transmissions within range of the receiver as well as, in the case of relocation or portable operation, the assigned frequencies in a different geographical region.
The versatility of scanning receivers was greatly enhanced with the advent of frequency synthesizers for generation of local oscillator frequencies. The frequency synthesizer is controlled by frequency codes stored in memory, and typically the memory can be reprogrammed for operation on different frequencies. It is also known to store a large number of frequency codes in memory, or alternatively to generate consecutive frequency codes as a function of known channel spacings. Increasing the number of tunable frequencies reduces the need for reprogramming, but it can also cause delays in tuning and increase the susceptibility to spurious signals commonly referred to as "birdies".
Delays in tuning occur because finite time is required for a frequency synthesizer to lock onto each new frequency and for a squelch circuit to detect presence or absence of activity on the tuned frequency. If there are many tunable frequencies and the receiver stops on each one, the time delay between successive stops on a given channel of interest can become excessive even if no other channel is active.
Birdies are internally generated signals which conventional scanning receivers recognize as active frequencies. This false recognition causes conventional scanners to stop scanning and remain tuned to that frequency, even though no audio modulation is present. When a scanner tunes to a momentarily inactive or locally unused frequency, a time delay is introduced, as mentioned above, but scanning resumes afterward. In the case of birdies, however, the scanner locks up and external intervention is required to release the scanner for further scanning.